1. Field of the Invention
The present invention relates to an induction heating device and a method of induction heating. More particularly, the present invention relates to an induction heating device and a method of induction heating used for local heating of a rolling roller of a rolling mill.
2. Description of the Related Art
In general, in a hot rolling process, a temperature difference occurs between a portion of a rolling roller in the roller width direction in which a rolled sheet passes and a portion of the rolling roller in the roller width direction in which the rolled sheet does not pass. Therefore, a difference in the roller diameter is caused in the roller width direction by the difference in the thermal expansion of the roller. Therefore, a hot rolling mill, which use an induction heating device for locally heating a low temperature portion of the rolling roller under the condition of non-contact so that the thermal expansion of the rolling roller can be kept constant in the roller width direction, was developed, as disclosed, for example, in JP2000-225406.
FIG. 7 is a schematic illustration showing an outline of a sheet rolling mill provided with a conventional induction heating device. In the sheet rolling mill, there are provided a pair of work rollers 1, 2 which are arranged opposed to each other. Also, there are provided a pair of backup rollers 3, 4 which are arranged in an upper and a lower portion. The sheet 5 to be rolled is inserted between the work rollers 1, 2 and hot-rolled and drawn out in the direction of arrow X. In this process of hot-rolling, the work rollers 1, 2 are thermally expanded by the heat of the sheet 5 to be rolled. Therefore, what is called a heat-crown is generated in which the roller diameter distribution in the roller axis direction becomes maximum at the center of the roller axis.
When hot-rolling is continued under the condition where a heat-crown is generated, the sheet thickness becomes unequal, and the quality of the rolled sheet deteriorates. In order to solve the above problems associated with a heat-crown, there are provided four induction heaters 6 on the delivery side (alternatively on the entry side) of the work rollers 1, 2 in such a manner that the induction heaters 6 can slide on the sliding rails 7 which are arranged in parallel with the axes of the work rollers 1, 2, while the induction heaters 6 are arranged opposed to the work rollers 1, 2. These induction heaters 6 are supplied with electric power by the electric power source unit 8xe2x80x2.
As shown in FIG. 8, each induction heater 6 is composed as follows. In a hollow portion of the coil 61, for example, there are provided ferromagnetic body cores 62 (ferrite), the profile of each of which is a rectangular parallelepiped, and also there are provided water-cooled plates 63 to cool the cores, wherein the ferromagnetic body cores 62 and the water-cooled plates 63 are alternately arranged in contact with each other. Cooling water is supplied from the cooling water supply unit 66 to each water-cooled plate 63 via the pipes 65, 64. Using the above structure, the occurrence of overheating caused by heat generated by the coil 61 and the ferromagnetic body cores 62 can be prevented.
This induction heating device reads the width of the sheet, reads the temperature of the sheet, reads a target profile of the rolled sheet, estimates current and future roller profile and controls the heating quantity and the heating position of the rollers so that an optimum sheet profile can be obtained after rolling.
In the above apparatus of the prior art, the quantity of heat given by the induction heater 6 is controlled when an electric current or voltage is controlled by the electric power supply unit 8xe2x80x2, and the frequency of electric power is not controlled. Therefore, the following problems may be encountered.
(1) When the frequency of electric power supplied to the coil 61 is low, the electric current generated on the surface of a roller penetrates into a deep portion of the roller. Therefore, not only the surface of the rolling roller 1, 2 but also the inside is heated. As a result, the heating density is lowered. Accordingly, the heating efficiency, which is necessary for correcting the thermal expansion of the rolling roller 1, 2, is lowered. Further, since the magnetic flux density in the core 62 tends to increase when the frequency is low, the size of the induction heater 6 necessary for obtaining a predetermined quantity of heat is increased.
(2) On the other hand, when the frequency of electric power supplied to the coil 61 is high, the high frequency loss of a feeder is increased, as is the loss of a matching circuit including the core 62, and the voltage impressed upon the coil. As a result, the electric power transmission efficiency is lowered and electric breakdown tends to occur due to the high voltage.
(3) Electric power outputted from the electric power supply unit 8xe2x80x2 is adjusted by adjusting the electric current or voltage. Therefore, it is not possible to reduce the size of the electric power supply unit 8xe2x80x2 significantly.
It is an object of the present invention to provide an induction heating device for heating a rolling roller by induction heating and a method of induction heating by which the diameter of the rolling roller can be stably equalized.
According to the present invention, there is provided an induction heating device for heating a rolling roller by induction heating so as to equalize the diameter of the rolling roller, comprising:
an induction heater; an induction heater moving means for moving the induction heater to a heating portion while a distance from the induction heater to the surface of a portion to be heated is kept constant; an electric power supply means for supplying electric power to the induction heater; and a heat quantity adjusting means for adjusting the quantity of heat of the induction heater according to the quantity of heat required by the heating portion, the heat quantity adjusting means including a frequency control means for controlling the frequency of electric power supplied to the induction heater, wherein the frequency control means adjusts the frequency of electric power to be high in a predetermined frequency range when the quantity of heat required by the heating portion is large, and adjusts the frequency of electric power to be low when the quantity of heat required by the heating portion is small.
According to the present invention, there is also provided an induction heating method of heating a rolling roller by an induction heating device so as to equalize the diameter of the rolling roller,
the induction heating device comprising: an induction heater; an induction heater moving means for moving the induction heater to a heating portion while the distance from the induction heater to the surface of a portion to be heated is kept constant; an electric power supply means for supplying electric power to the induction heater; and a heat quantity adjusting means for adjusting the quantity of heat of the induction heater according to the quantity of heat required by the heating portion,
the induction heating method comprising the steps of: adjusting the frequency of electric power to be high in a predetermined frequency range by the frequency control means when the quantity of heat required by the heating portion is large; and adjusting the frequency of electric power to be low by the frequency control means when the quantity of heat required by the heating portion is small.
The present invention can be more fully understood from the description of preferred embodiments of the invention set forth below, together with the accompanying drawings.